1. Field of the Invention
The present invention relates to a charged particle beam apparatus for observing and inspecting the surface of a sample, such as semiconductor wafer or photomask, which is liable to be contaminated by irradiation with a charged particle beam and to have the image observation spoilt, and a contamination removal method therefor.
2. Description of the Related Art
Heretofore, in an electron beam apparatus such as scanning electron microscope (SEM), it has been well known that image observation is hampered by the contamination of a sample attendant upon irradiation with an electron beam (refer to Non-patent Document 1). The contamination is said to be ascribable to the fact that the electron beam will impinge against hydrocarbons floating or adsorbed in the surface of the sample, to turn the hydrocarbons into carbon and to deposit the carbon on the sample. It is considered that much of the hydrocarbons will, not only be produced by gases emitted from the inside components of the apparatus, but also be brought into the SEM by the sample having already been contaminated. It is therefore often observed that the production rate of the contamination becomes much higher than usual.
As countermeasures against the contamination in the electron beam apparatus, there are the following examples:
(1) Low-temperature Contamination Prevention Apparatus: A metal plate held at a low temperature (for example, liquid nitrogen temperature) is disposed around a sample which is irradiated with an electron beam, so as to adsorb hydrocarbons into the metal plate and to diminish the contamination of the sample.
(2) Purification and Degassing of Components: Components inside a sample chamber are subjected to ultrasonic cleaning with a solvent and are further degassed at high temperatures, and they are thereafter assembled into an electron beam apparatus, whereby hydrocarbons to be emitted are decreased, and the contamination of a sample is diminished.
Even after the above countermeasures (1) and (2), the diminution of the contamination of the sample is sometimes unsatisfactory. Especially in the observation of the surface of a semiconductor wafer or photomask or a pattern length measurement on the surface, an identical place is measured a plurality of times. On this occasion, a pattern size is often changed by the contamination attendant upon the electron beam irradiation, and even when the magnitude of the change is slight, unallowable lowering in the reproduction precision of length measurement values is sometimes incurred.
(3) Down-flow type Asher: Active oxygen is produced by radio-frequency discharge from a mixture gas consisting of O2 and CF4 and is reacted with hydrocarbons, thereby to remove contamination (refer to Non-patent Document 2). With this technique, an optimization control is difficult, and rather the lowering of the reproduction precision of length measurement values attributed to etching will be incurred in the observation of the surface of a semiconductor wafer or photomask or a pattern length measurement on the surface.
As stated above, it cannot be said that the related-art countermeasures against the contamination in the electron beam irradiation apparatus are satisfactory.
Meanwhile, in semiconductor manufacture, a dry cleaning method wherein organic substances on the surface of an Si substrate are removed by irradiation with ultraviolet rays has been well known. The principles of this method are as stated below. Oxygen O2 is dissociated into active oxygen O by the ultraviolet rays. Owing to the active oxygen, the organic substances undergo oxidation decompositions, thereby to be volatilized and removed. In particular, it has been known that a method of cleaning the Si substrate by irradiation with ultraviolet rays (vacuum ultraviolet rays at a wavelength of 172 nm) from an excimer lamp is effective (refer to Non-patent Document 3). This document indicates that, in the atmospheric air, when the density of the active oxygen at the sample surface is heightened by setting several mm or less as the distance between the sample and the window plane of the excimer lamp, a cleaning effect increases, whereas when the distance is made longer, the cleaning effect decreases because an ultraviolet dose to fall on the sample surface lessens due to the absorption of the ultraviolet rays by the atmospheric air, so the quantity of the active oxygen to appear in the vicinity of the surface lessens.
It is to be noted, however, that the ultraviolet irradiation has never been employed for the removal or prevention of the contamination in the charged particle beam apparatus.
[Non-patent Document 1] Electron Microscope (1981), Vol. 16, No. 1, p. 2, published by the Japanese Society of Microscopy
[Non-patent Document 2] Materials of the 117th Study Meeting (1991), p. 137, 132nd Committee, published by Japan Society for the Promotion of Science
[Non-patent Document 3] Paper Issue (1999), Vol. 83, No. 5, published by the Illuminating Engineering Institute of Japan